1. Field of the Invention
The present invention relates both to a hydrogen generator configured to reform a hydrocarbon-based feed gas such as a city gas or a LP gas using steam to generate a reformed gas containing hydrogen as major component, and to a fuel cell power generation system comprising the hydrogen generator.
2. Description of the Related Art
A hydrogen generator configured to steam-reform a hydrocarbon-based feed gas such as a city gas or a LP gas to generate a reformed gas containing hydrogen as major component is used to, for example, generate hydrogen for use as a feed gas in a fuel cell. Such a reaction is called a steam reforming reaction or a reforming reaction. Since the reforming reaction in the hydrogen generator is an endothermic reaction, a reformer must keep its temperature between approximately 550 to 800° C. to maintain the reforming reaction To this end, in the hydrogen generator, a heat source such as a burner is installed to heat the reformer using a high-temperature combustion gas derived from the heat source, or heat radiator that emits radiation heat of the combustion gas.
The reformed gas obtained in the reformer of the hydrogen generator contains CO generated as by-product in the reforming reaction, as well as hydrogen as major component. If the reformed gas containing GO is directly supplied to the fuel cell, then CO degrades activity of a catalyst within the fuel cell. In order to remove CO, the hydrogen generator is provided with a Shifter and a Purifier located downstream of the reformer to shift CO contained in the reformed gas into CO2.
For the purpose of conducting the shift reaction efficiently, the Shifter of the conventional hydrogen generator is set at a temperature between 180 to 400° C., which is an optimum temperature for the shift reaction. In order for the Shifter to be kept at such a temperature, heat recovered from the reformed gas of a temperature between 550 to 800° C. which is generated in the reformer is used to heat the Shifter, while the Shifter is cooled by heat exchange between a combustion gas (combustion off gas) after use as the heat source to heat the reformer and the Shifter (see Japanese Laid-Open Patent Application Publication No. 2002-25593, page 4 to 7, FIG. 1), or the Shifter is cooled by heat exchange with a combustion fuel gas used in the heat source such as the burner or air (see Japanese Laid-Open Patent Application Publication No. 2002-187705, page 5 to 10, FIG. 1).
In the conventional hydrogen generator constructed as described above, heat recovered from the Shifter by the combustion gas, the combustion fuel gas or the air to allow the Shifter to be cooled, is not transmitted to the feed material or the steam, and thus, efficient use of such recovered heat has not been achieved. For this reason, it is difficult to return a substantially total heat to the reformer, and therefore, sufficiently high heat efficiency is not achieved.
In the above construction, temperature of the fuel off gas, a flow rate of the combustion fuel gas or the air varies if a load of hydrogen generation in the reformer varies. When conditions of the combustion off gas, the combustion fuel gas, and the air vary, the amount of heat recovered from the Shifter correspondingly varies. For this reason, it is difficult to control the amount of heat recovered from the Shifter. As a result, since the Shifter is not kept at an optimum temperature, ability of the Shifter to remove CO is not sufficiently high. And, if the gas containing a substantial amount of CO is supplied to the fuel cell in the fuel cell power generation system, performance of the fuel cell would be degraded.